A light emitting diode (LED) is a semiconductor element which converts electrical energy into light such as ultraviolet light or visible light to emit the light, and has advantages that it has a long life and high reliability, and when it is used as a light source, the time which exchanges lamps can be saved. An LED lamp having an LED chip sealed with a transparent resin is used extensively for the backlight of a liquid crystal display device used for a display portion of portable communication devices, PC peripheral devices, office automation equipment, home electric appliances and the like, and also for the illuminating device of signal equipment, switches, vehicle-mounted lamps, general lighting and the like.
As to the color tone of the light irradiated from the LED lamp, light having a visible light region ranging from blue to red suitable for usages can be realized by combining an LED chip and phosphors having various emission colors. Especially, a white light-emitting type LED lamp (white LED lamp) is spreading quickly for usage as the backlight of the liquid crystal display device, vehicle-mounted lamps, and the like. Its use is expected to be increased drastically as a replacement for the fluorescent lamp in the future. For example, since mercury is used for the ordinary fluorescent lamp, it is considered that a white LED lamp not using mercury takes the place of the fluorescent lamp in the future.
At present, an LED lamp combining a blue light emitting LED and a yellow phosphor (such as YAG), and an LED lamp combining an ultraviolet LED having a light emission wavelength of 360 to 440 nm and a mixture (BGR phosphor) of blue, green and red phosphors are known as the white LED lamps which are becoming popular or being tried. The former is becoming more popular than the latter because its luminance characteristics are superior to the latter at present. But, the former white LED lamp has light distribution which is biased toward a blue component and a yellow component, and has insufficient light of a red component. Therefore, even if the former white LED lamp has target emission chromaticity as a light-source light, it has a disadvantage that when this light source is used to see a subject, the reflected light is considerably different from the natural color visible in sunlight.
On the other hand, the latter white LED lamp using the ultraviolet LED is inferior in luminance to the former, but the emitted light or the projected light does not have a heavy irregular color, and it is expected to become the mainstream of the white lamp in the future. As to the white LED lamp using the ultraviolet LED, the improvement of the lamp characteristics such as luminance (brightness), color rendering properties and the like is under way based on the characteristics of the phosphors and their combinations (see Patent References 1, 2). For example, to improve the brightness of the white LED lamp, it is considered to use the yellow phosphor having an emission peak wavelength of 540 to 570 nm instead of the green phosphor having an emission peak wavelength of 500 to 530 nm.
The white LED lamp applying a mixed phosphor (BYR phosphor) containing the yellow phosphor instead of the green phosphor is expected to be a light source for the illuminating device because its brightness is improved more than the white LED lamp using the BGR phosphor. But, the conventional white LED lamp applying the BYR phosphor containing the yellow phosphor is not necessarily improved in its properties and, therefore, it is demanded to be improved more in the luminance, color rendering properties and the like. Meanwhile, for the yellow phosphor, there are proposed various phosphors which are used in combination with the blue light emitting LED.
As the yellow phosphor used in combination with the blue light emitting LED, there are known a cerium-activated yttrium aluminate phosphor (YAG), a cerium-activated terbium aluminate phosphor (TAG), and a europium-activated alkaline earth silicate phosphor (BOSS) (see Patent Reference 3). As to the conventional yellow phosphor, its light-emitting property when excited by the blue light (a light emission wavelength: 430 to 500 nm) emitted from the blue light emitting LED is being studied, but its light-emitting property when excited by the light having a light emission wavelength of 360 to 440 nm emitted from the ultraviolet LED is not studied fully. Therefore, its study and improvement of the light-emitting property are being demanded.    Patent Reference 1: JP-A 2002-171000 (KOKAI)    Patent Reference 2: JP-A 2003-160785 (KOKAI)    Patent Reference 3: JP-B2 3749243 (Patent Registration)